Method of acid treating wells



United States Patent 3,463,232 METHOD OF ACID TREATING WELLS George E.Scofield, 19 Wakeman Road, Darien, Conn. 06820 No Drawing. Filed Nov. 3,1967, Ser. No. 680,327 Int. Cl. E21b 43/27 U.S. Cl. 166307 6 ClaimsABSTRACT OF THE DISCLOSURE The productive capacity of a well is greatlyincreased by injecting a dilute acid solution containing the equivalentof from about 0.00003 percent by weight to 0.5 percent by weighthydrochloric acid continuously into the well for a period of at leastseveral hours. When so treated, the solution scours and enlarges thepores of the formation for a substantial distance from the well, and notmerely in the vicinity of the well bore.

BACKGROUND OF THE INVENTION Field of the invention This inventionrelates to a method for treating a well in order to increase itsproductive capacity by injecting an aqueous solution of an acid into thewell, thereby enlarging the pores of the acid-soluble materialsurrounding the well bore.

Description of the prior art It has been previously though necessary touse strong solutions of acids to treat wells, as exemplified by thesolutions containing from about to about 35% by weight hydrochloric acidemployed in the processes disclosed in US Patents Nos. 2,038,956,2,308,414 and 2,640,810. In these prior art methods, the strong acidsolutions tend to attack the tubing and other metal parts of the wellcausing serious operating and maintenance problems. Further, when thetreatment of a well is attempted by injecting a strong acid inconjunction with an inhibitor to retard metal attack, the desiredenlargement of the pores of the well formation is often not obtained.Treatment with the strong acid solutions described in the prior art hasthe further disadvantage that the increased permeability of theacid-soluble material of the formation has been restricted to the areaimmediately surrounding the well bore, as disclosed in US. Patents Nos.2,308,414 and 3,160,207. Moreover, the use of the strong acid solutionsof the prior art limits the shut-in time during which the acid canattack the well formation to about 3 or 4 hours, as disclosed in US.Patent No. 2,640,810. Other means of treating wells include wellfracturing, as disclosed in US. Patent No. 3,104,706, and the methods ofacidization and fracturing are sometimes combined.

SUMMARY OF THE INVENTION The present invention is based upon mydiscovery that the injection of a large volume of a dilute acid solutioninto a well, when continued for a period of at least several hours,greatly increases the productive capacity of a well. Specifically, Ihave discovered that the injection of an aqueous solution containing theequivalent of from about 0.00003 to about 0.5 percent by weight ofhydrochloric acid for a period of at least several hours, and preferablyfor twelve or more hours, extends the beneficial treatment deep into thewell formation and thereby greatly increases the productivity of thewell as compared with the acidization processes of the prior art. Theimproved results obtained appear to be due to the fact that PatentedAug. 26, 1969 "ice DESCRIPTION OF THE PREFERRED EMBODIMENT The practiceof my invention is exemplified by the advantageous embodimenthereinafter described.

A portable pump having a capacity that roughly approximates the desiredcapacity of the well is connected to an available source of water. Forexample, if the well to be treated has shown a decline in capacity from2000 g.p.m. to 500 g.p.m., it would be desirable to select a pump with acapacity of about 2000 g.p.m. The discharge of the pump is connected tothe well to be treated, and if this is a well that is equipped with apump, the well pump is preferably removed, A connection at the dischargeof the pump provides water to operate an eductor which drawsconcentrated acid from a tank-car and injects it by means of a spargerinto the main stream of Water being pumped into the well. Recordinginstruments comprising, for example, a flowmeter, a pressure gage, and apH meter, enable the operators to control the acidity of water pumpedinto the well and to observe the progress of the treatment. In one formof the invention, the flow of concentrated acid is regulated in anautomatic manner by the pH meter so as to continuously provide thedesired pH in the water being pumped into the Well.

The beneficial effects of the dilute acid can be continuously observedin the response of the flowmeter and pressure gage. At the start oftreatment the rate of flow will be relatively low and the solutionpressure will be relatively high, and the quotient of solution flowdivided by solution pressure will be at a minimum. As treatment proceedsand the pores in the formation become larger, the flow will increase andthe pressure will decrease leading to an increasing flow to pressureratio. The percentage increase in this quotient or ratio is a roughindication of the percentage increase in the capacity that is beingobtained for the Well. The treatment preferably should be continued aslong as the quotient of flow divided by pressure continues to show asignificant increase. After treatment has been carried out to anacceptable degree, the procedure is terminated and if the well is thepumped type, its pump is reinstalled. The well is then allowed to flowto Waste, or is pumped, or is drained by other known means, until thewater is sufficiently free of acid and dis solved salts to be acceptablefor use.

The optimum combination of treatment conditions that will lead to anincrease in well capacity varies with the cost of concentrated acid, thequantity of dilution water available, the time available to carry outthe treatment, and the reactivity of the walls of the pores in theformation to the acid. As a general rule, the longer the treatment canbe carried out, and the higher the flow of dilute acids that can beforced into the formation, the greater is the improvement that can beexpected, treatment times of twelve hours or longer generally beingpreferred.

The acidity of the water that is pumped into the well formation shouldbe maintained between a pH of 1 and 5. Although any clean acid, such asnitric acid, may be used to achieve this pH range, hydrochloric acid isusually the least expensive and most convenient acid to use, Theconcentration of hydrochloric acid in a solution having a pH withinthese limits would range from about 0.5 percent to as low as 0.00003percent by weight ttuuut HCl, and the use of other acids would requirethe presence of equivalent amounts of these acids in the water. In mywork I have found that a pH of about 3 is an effective level of solutionacidity.

The rate at which the dilute acid is pumped into the formation should beas high as possible, for in this way the beneficial scouring efiect isachieved and, moreover, the acid is carried far back in the formationbefore it has expended itself. A rule-of-thumb determination would callfor a flow rate of dilute acid that approximates the flow that isexpected to be achieved from the well after the treatment. Lesser flowrates for the dilute acid will be less effective. In any case, the rateof flow should not be allowed to drop below 1 percent of the flow thatis expected to be achieved from the well after treatment.

It has been found that the duration of treatment may continue for aperiod of several days without any harmful corrosive effect beingobserved on the tubing or other metal portions of the well equipment.The actual duration of treatment will obviously depend upon the rate ofinjection of the dilute acid solution and the measured observableresponse from the flowmeter and the pressure gage. In general, thetreatment is best terminated when the ratio of solution flow to solutionpressure no longer shows any significant change.

I claim:

1. In the method for increasing the productive capacity of a well bytreating the well with an aqueous solution of an acid, the improvementwhich comprises injecting a large volume of a dilute acid solution intothe well under pressure, and continuing injection of said dilute acidfor a period of at least several hours, the concentration of acid insaid solution being maintained at that level which is equivalent to ahydrochloric acid concentration within the range of from about 0.5percent by weight to 0.00003 percent by weight hydrochloric acid, andthe pH of the acid solution being maintained between about pH 1 and pH5.

2. The method according to claim 1, wherein the treatment with said acidsolution is continued for a period of at least twelve hours.

3. The method according to claim 1, wherein the injection of the acidicsolution into the well is continued throughout the period during whichthe quotient of solution flow divided by solution pressure continues toshow a significant increase.

4. The method according to claim 1 wherein the pH of the solution ismaintained at about 3.

5. The method according .to claim 1, wherein the rate of flow of theacidified solution into the well is at least 1 percent of the flow ofthe Well after said treatment thereof.

6. The method according to claim 1, wherein hydrochloric acid isemployed to acidify the treatment solution.

References Cited UNITED STATES PATENTS STEPHEN I. NOVOSAD, PrimaryExaminer

